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Classes representing the components of an XML document. 
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In addition there are the Exceptions related to JDOM processing and some classes
useful for creating and accessing JDOM Content.
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<h1>Core JDOM classes</h1>
All XML in JDOM is represented in the following classes:
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<li> Text - regular parsed XML character content (PCDATA).
<li> CDATA - unparsed XML text content (can contain &lt; &gt; and &amp;). Note: in JDOM, CDATA class exends Text.
<li> Comment - XML Comments
<li> EntityRef - Entity References (e.g. &amp;refeg; )
<li> ProcessingInstruction - As the name suggests
<li> DocType - The relevant details of any DOCTYPE Declaration.
<li> Element - An XML element 
<li> Document - A representation of a complete XML document
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In addition to these 8 classes there are also the Attribute and Namespace
classes which are used to represent these respective XML structures in Element.
In the DOM model the 'Attr' (attribute) class is considered to be a DOM 'Node'.
In JDOM this is <strong>not</strong> the case - Attribute is not Content.
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The XML Structure is embodied in the the concept of Parent JDOM classes and
regular JDOM Content. Parent is an interface, and Content is an abstract class.
The Document and Element classes are both Parent classes, Text, CDATA, Comment,
EntityRef, ProcessingInstruction, DocType and Element are all Content. Note that
Element is both Parent and Content.
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To enforce XML well-formedness, Document is only allowed a restricted set of
child content: any number of ProcessingInstructions and Comments, one DocType,
and one Element (the 'root' element). Element is allowed any child content
except DocType.
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The NamespaceAware interface identifies those JDOM constructs which are
sensitive to Namespaces, which is all 8 core types and also Attribute. In JDOM
NamespaceAware classes are able to identify and report the Namespace Context in
which they exist.
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<h1>JDOM helper classes</h1>
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The Verifier is a special class useful in ensuring well-formedness of documents.
It contains all the rules for ensuring the JDOM model always has well-formed
content.
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JDOMConstants interface contains a number of constant values that JDOM users may
find useful when creating or manipulating JDOM structures. These are in
addition to (but some may duplicate) the constants found in the
javax.xml.XMLConstants class.
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The JDOMFactory interface is primarily used when building JDOM documents from
some source (SAX, DOM, etc.) using one of the input Builders (SAXBuilder,
DOMBuilder, etc.). The default JDOMFactory is the DefaultJDOMFactory).
If you have custom JDOM classes or want special treatment for content as it is
being created you can supply you own JDOMFactory instance to the input Builder.
Typically you would extend the DefaultJDOMFactory for this purpose. 
The DefaultJDOMFactory ensures all XML rules are followed correctly. The
UncheckedJDOMFactory may create JDOM content that does not follow XML
well-formedness rules. Use the UncheckedJDOMFactory in places where you are
certain the input is correct (perhaps the results of a document parsed by a 
trusted third-party parser). The UncheckedJDOMParser is only marginally faster
than the DefaultJDOMParser.
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